1. Field
The present invention relates to an active matrix display device in which a control signal driving the active matrix display device can be continuously supplied to an image signal line driving unit of the active matrix display device even during a vertical blanking period.
2. Description of the Related Art
In a driving circuit for an image display device, such as a liquid crystal display device like an active matrix display device, similar to a vertical scanning effective display period, a control signal is continuously supplied to an image signal line driving unit even in a vertical blanking period. This enables image signal lines (source lines) of the display device is maintained in a state similar to a driving state. This is effective in that irregularities can be prevented from occurring in display on a display screen for each horizontal line.
As such, in order to continuously supply the control signals to the image signal line driving unit during the vertical blanking period, the respective control signals need to be transmitted at the same timing (period) as the timing during a vertical scanning period or the similar timing to the timing during the vertical scanning period. Further, a timing of a horizontal synchronizing signal that is generated during the vertical blanking period needs to be the same as or similar to the timing of a signal generated during a vertical scanning effective display period. JP-A-2003-91266 discloses an image display device including a horizontal reference signal generating circuit that generates a pseudo horizontal reference signal during the vertical blanking period (for example, FIG. 6 of JP-A-2003-91266).
However, when a synchronizing signal input by an external signal source (for example, computer main unit side or the like) varies and the length of the vertical blanking period varies, a control signal that is transmitted from a display control unit to an image signal line driving unit during the vertical blanking period interferes with a control signal that is transmitted from a timing control unit to the image signal line driving unit during a display period of a next frame after the vertical banking period is completed. As a result, the image signal line driving unit may cause an erroneous operation.
Accordingly, a driving control signal is not transmitted by a final portion of a vertical blanking period (corresponding to about one to two horizontal periods) (for example, FIG. 9 of JP-A-2003-91266).
However, cutting a signal generated for the purpose of making the image signal line of the display device entering a driving state becomes a hindrance factor with respect to achieving the object. The signal needs to be appropriately cut during a period as short as possible. In particular, during a period until the vertical blanking period is completed, if a driving operation of the image signal line pauses for a long time, it has a large influence on a vertical scanning period of a next frame.
Further, according to the above-described method, a counter needs to be provided to count a horizontal period of the vertical blanking period. The length of the vertical blanking period varies according to systems that input signals to the liquid crystal display device. For this reason, the counter needs to estimate various signals input to the liquid crystal display device, such that the counter copes with the considered maximal counter number. Therefore, a relatively large-scaled circuit needs to be provided (for example, FIG. 17 of JP-A-2003-91266).
Further, since a value counted by the counter is used in a next frame, it is not possible to cope with external input signals in which the length of a vertical blanking period varies for each frame.